[Ruby] Operator ":: (colon colon)" [constant]

Preface

If you're using Rails, you've probably seen the following description:

models/application_record.rb

class ApplicationRecord < ActiveRecord::Base
#Omitted below

You inherited ActiveRecord :: Base from Application Record, right?

Nothing has changed.

・ ・ ・

・ ・

・

Well, eh, wait a minute. What is it, ::! ?? !! ?? !! ?? scared! !! !!

So, as a Ruby beginner, I will leave a memorandum of research on ::.

Conclusion

:: has two features.

1. Specify the absolute path of the namespace

module Hoge
  class Fuga
    def self.fuga
      p "fuga"
    end
  end
end

Hoge::Fuga.fuga #=> "fuga"
Fuga.fuga #error

#Create Fuga class
class Fuga
  def self.fuga
    p "fuga"
  end
end

Hoge::Fuga.fuga #=> "fuga"
Fuga.fuga #=> "fuga"

2. Constant scope operator

Use :: to reference constants defined in a class or module from outside the scope.

class Hoge
  FUGA = "FUGA" #Constant FUGA
  
  p FUGA #=> "FUGA"
end

#Output constant FUGA from outside the scope.
p Hoge::FUGA #=> "FUGA"

Also, ** Ruby treats classes and modules as constants. ** ** For example, when you create a Hoge class, the Hoge class object is assigned to a constant ** named ** Hoge.

After all, what is the :: of ActiveRecord :: Base? ??

The :: in ActiveRecord :: Base points to the ** Base class in the ActiveRecord module! !! ** ** It's a Base class nested in ActiveRecord. The simple description is as follows.

active_record/base.rb

module ActiveRecord

#abridgement

  class Base
  #abridgement
  end

#abridgement

end

The official contents of ActiveRecord :: Base are as follows. https://github.com/rails/rails/blob/master/activerecord/lib/active_record/base.rb

This is a deeper summary of ActiveRecord features. Rails: ActiveRecord :: Base Method Summary

reference

Official Ruby Reference Variables and Constants https://docs.ruby-lang.org/ja/latest/doc/spec=2fvariables.html#const

Rails Guide Active Record Basics https://railsguides.jp/active_record_basics.html